Individuals with lower limb amputations generally have greater difficulty maintaining balance, especially when walking on uneven surfaces. Seventy-three percent of outdoor falls are due to environmental factors such as uneven surfaces on sidewalks, curbs, and streets (Li et al., 2006). A fall outdoors is three times more likely to result in an injury (Kelsey et al., 2012), which may contribute to the fear of falling and reduced activity levels (Miller et al., 2001, Miller and Deathe, 2011). Thus, maintaining balance when walking on uneven surfaces is important for participation in community activities.

Walking on uneven surfaces requires several biomechanical responses by the lower limbs to conform to the surface and maintain balance. Individuals without mobility impairments can navigate uneven surfaces by reducing step width after a disturbance (Dixon and Pearsall, 2010, Yeates et al., 2016), increasing knee and hip flexion (Gates et al., 2012), and increasing ankle and hip power (Panizzolo et al., 2017, Segal et al., 2018). Individuals with lower limb amputations likely have a harder time adapting to uneven surfaces due to a lack of ankle-foot muscle actuation and the passive characteristics of their prosthetic foot. Most currently prescribed multiaxial prosthetic feet for individuals with transtibial amputation (TTA) are passive energy-storing-and-returning feet. These types of feet aim to improve the performance and activity level of individuals with TTA by attempting to mimic the shock absorption and propulsion of the intact foot (Hafner et al., 2002). However, these prosthetic feet can only return as much energy as they absorb, and act passively in all axes of deformation, which may not be desirable over certain uneven surfaces. Different types of uneven surfaces may even further decrease the propulsive output of these prosthetic feet. Understanding how individuals with TTA respond to uneven surfaces while wearing their currently prescribed prosthetic feet, compared to individuals without mobility impairments, can provide insight into possible prosthetic or therapeutic interventions that minimize needed biomechanical responses.

Cross-slopes can be challenging even for individuals without mobility impairments. For example, adult males taking several steps on a 10% grade cross-slope walked with increased hip adduction on the up-slope limb, and decreased hip adduction on the down-slope limb (Dixon and Pearsall, 2010). During prolonged cross-slope walking, individuals with TTA have been shown to increase their prosthetic knee and hip flexion during swing, and decreasing their contralateral knee flexion during stance (Villa et al., 2017). When stepping on a coronally uneven and unpredictable surface, individuals with TTA wearing a novel, coronally clutching ankle (i.e., conformal inverting and everting) showed improvement in some aspects of balance, such as center of mass path regulation, but not all when compared to their prescribed prosthesis (Yeates et al., 2018). Yet rarely are individuals with TTA trained on how to walk on uneven surfaces (Matjaĉić and Burger, 2003, Sjödahl et al., 2001). Current research has yet to explore the biomechanics of individuals with TTA taking a single step on a coronally uneven surface while wearing their prescribed prosthesis. A transient, single step on a coronally uneven surface, simulating unexpected variations in natural and manufactured terrain, may present a greater challenge to balance for individuals with TTA compared to individuals without mobility impairments.

The objective of this research was to quantify the biomechanical response of individuals with a unilateral TTA wearing their clinically prescribed prosthesis versus individuals without mobility impairments (controls), during and after a single step on a coronally uneven and unexpected surface. A single step was chosen to allow observation of the biomechanical response without further disturbances during subsequent recovery steps. A novel experimental design was used to produce three different terrain conditions: unblinded flush (flat surface), 15˚ blinded inversion, and 15˚ blinded eversion. The 15˚ coronal angle was chosen to produce an observable effect from the disturbance without causing injury to the participants (Yeates et al., 2016). Lower limb joint angles and vertical center of mass (COM) were calculated to give insight into lower limb joint strategies and biomechanical responses to uneven terrain. Joint work was calculated to give insight into energy expenditure and the effects of the prosthetic foot on propulsion over uneven terrain. Finally, recovery step width and vertical ground reaction force (GRF) impulse were calculated to give insight into balance and body support for individuals with TTA compared to controls. These findings may contribute to current rehabilitation practices for individuals with TTA and to future prosthesis design.